This invention relates to a press load meter which measures and displays the maximum value of dynamic stress caused during the operation of a press.
By the provision of a load meter on a press, the following various advantages can be obtained:
(1) The amount of feed can be controlled so that the load is most suitable with a metal mold set.
(2) Abnormal conditions such as engagement of foreign matters and wearing of a metal mold can be detected.
(3) Accordingly, the quality of work pieces can be controlled.
(4) Protection of the press can be suitably effected.
Therefore, one of the most important conditions for the load meter provided on a press is to readily measure a load value with high accuracy.
Furthermore, the following specific features are required for the press load meter:
(A) The maximum value of dynamic stress can be displayed for a relatively long period of time after the completion of a measurement.
(B) It can measure the load values at a plurality of points (two through four points) on the ram (slide) of the press as well as the sum of the load values.
(C) It has stability for a relatively long period and also simplicity in handling.
In a conventional press load measurement, when a press is operated to apply load to a work piece, strains are caused at various parts of the press by the load. The strains are proportional to the load. Therefore, strain gauges are provided at various measurement points, and Wheatstone bridges are provided for the measurement points, respectively. In addition, before starting the measurement, under the condition that no load is applied the unbalance outputs present in these Wheatstone bridges (hereinafter referred to as "initial values" when applicable) are zeroed by means of an initial balance controller. Thereafter, load is applied to the press, and the outputs of the Wheatstone bridges are measured with meters thereby obtaining the magnitudes of strains at the measurement points, namely, the magnitude of the load.
In this case, in the conventional device the initial unbalance control takes time and is troublesome because it is carried out manually for every measurement point; that is, the conventional device is poor in practical operation. Furthermore, in the conventional device, in the measurement of the maximum value of load a capacitor is charged with the output of the bridge circuit, and the voltage of the capacitor is displayed by a meter. In other words, the maximum value of load to be measured is held in an analog mode. Accordingly, the conventional device is disadvantageous in that the display value is decreased with time. In addition, the sum of the load values at the measurement points is obtained by switching the connections of the Wheatstone bridges, which leads to the disadvantage of the conventional device that the switching and connecting operations are rather troublesome.
Moreover, the conventional device is disadvantageous in that as it is provided with only one charge circuit and only one meter, it is impossible to simultaneously measure load values at all the measurement points (the load values at all the measurement points, and the sum thereof) for every one stroke of the press.